Glucose levels can be measured from the bloodstream, or from the interstitial fluid (ISF), which surrounds the body’s cells. There is a delay in ISF glucose response to changes in blood glucose,¹ which is unlikely to impact routine day-to-day treatment decisions. The time lag  2.4 ± 4.6 minutes for adults and 2.1 ± 5.0 minutes for pediatrics.² ISF glucose and BG measurements taken simultaneously won’t always match, and, in fact, are likely to be different³. 

References

1. Rebrin K, Sheppard NF Jr, Steil GM. Use of subcutaneous interstitial fluid glucose to estimate blood glucose: revisiting delay and Sensor offset. J Diabetes Sci Technol. 2010;4(5):1087-1098. 

2. Alva S, Bailey T, Brazg R, et al. Accuracy of a 14-Day Factory-Calibrated Continuous Glucose Monitoring System With Advanced Algorithm in Pediatric and Adult Population With Diabetes. J Diabetes Sci Technol. 2022;16(1):70-77.

3. FreeStyle Libre flash glucose monitoring system User's Manual.